The Illinois River

The Illinois River is plagued with problems associated with point and non-point source pollution and has been for more than 200 years. In order to understand fully the sources of these pollutants and the impacts to the river, it is necessary to understand how the region’s geology has contributed to these water quality problems.

The Illinois River has two reaches with very different characteristics, a result of three major geological events: the east-west or upper reach begins at the confluence of the Des Plaines and Kankakee rivers; the north-south or lower reach begins at the “Big Bend” at Hennepin and flows to Grafton where it joins the Mississippi River (Cruikshank 1998).

Three significant geologic events shape the Illinois River valley: the first occurs during the next-to-the-last glaciation when glacial ice diverts the Mississippi River east to present-day Hennepin. The Illinois River bedrock valley expands as a result of this flow; next, approximately 21,500 years ago, during the Wisconsinan glacier, glacial ice moves west from Lake Michigan, blocks the Mississippi River, and diverts the Mississippi River to the west from Hennepin to Peoria. The pre-Wisconsinan glaciers continue to broaden and deepen the original Mississippi River, making it much deeper than is warranted by the volume of water that arrives as the glaciers retreat; finally, as the last glacier retreats, large meltwater lakes form behind the moraines west and southwest of Chicago. About 15,500-16,000 years ago, these glacial ice dams fail, releasing a tremendous flood, the Kankakee Torrent. This flood carves the upper Illinois River valley, widens and modifies the valley downstream of Hennepin, and pushes the Mississippi west to its present-day channel (Wetlands Initiative).